In https://codereview.chromium.org/2479113002/ it was discovered that
current codebase has two duplicate TestingPrefServiceSyncable classes.
One in component/sync_preferences and another in
components/pref_registry. The decision from discussion is to delete the
one in components/pref_registry.

BUG=672900

R=gab@chromium.org, vasilii@chromium.org, hashimoto@chromium.org, pkasting@chromium.org, rdevlin.cronin@chromium.org, ben@chromium.org, blundell@chromium.org

TBR=rdevlin.cronin@chromium.org, ben@chromium.org, blundell@chromium.org

Review-Url: https://codereview.chromium.org/2562733003
Cr-Commit-Position: refs/heads/master@{#437790}